In recent years, an electronic clock has greatly extended battery life by lowering current consumption of a step motor. The lowering of the current consumption of the step motor is achieved by usually driving the step motor with a small driving force that consumes a small amount of current, while driving the step motor with large driving force only when a rotor fails to rotate due to load increase. In a widely-used conventional method of detecting rotation/non-rotation of a rotor, a free vibration pattern of a rotor is determined as follows: after application of a normal drive pulse has terminated, a detection pulse is outputted to quickly change an impedance value of a coil of the step motor; and then an induced voltage generated in the coil is detected at an end of the coil. For example, firstly, one of two drive inverters respectively connected to two ends of the coil is operated to output a detection pulse as a first detection mode. Then, when a rotation detection signal is generated, the first detection mode is stopped while the other drive inverter is operated to output a detection pulse as a second detection mode. If a rotation detection signal is generated during the second detection mode, it is determined that the rotor is successfully rotated.
The second detection mode is for detecting that rotation has succeeded, i.e., that a rotor has exceeded a mountain of magnetic potential. The first detection mode is performed prior to the second detection mode to prevent detection of an erroneous detection signal generated before the rotor completely exceeds the mountain of magnetic potential when the rotor is driven relatively weakly. This prevents a waveform of a current waveform c2 of FIG. 17 or 18 from being erroneously determined as a detection signal showing that the rotor has exceeded the magnetic potential even though the rotation of the rotor has not yet terminated. Hence, performing the first detection before the second detection mode is known as an effective technique for more reliably detecting rotation. (Refer to Patent Document 1 and Patent Document 2, for example.)
Hereinbelow, the conventional technique is described with reference to the drawings. FIG. 15 is a block diagram showing a circuit configuration of a conventional electronic clock. FIG. 16 is a diagram showing pulse waveforms generated in the conventional electronic clock. FIG. 17 is a diagram of current waveforms and voltage waveforms generated in the coil when the rotor has successfully rotated. FIG. 18 is one example of a diagram of current waveforms and voltage waveforms generated in the coil when the rotor has successfully rotated.
In FIG. 15, reference numeral 20 is a step motor formed of a coil 9 and a rotor 10, reference numeral 1 is an oscillating circuit, reference numeral 2 is a frequency divider circuit, and reference numeral 3 is a normal drive pulse generating circuit which is configured to output ¾ ms-wide normal drive pulses SP every 1 ms in width of 5 ms, as shown in FIG. 16(a), at the beginning of every second, based on a signal of the frequency divider circuit 2. Reference numeral 4 is a correction drive pulse generating circuit which is configured to output a correction drive pulse FP of 10 ms, based on the signal of the frequency divider circuit 2 as shown in FIG. 16(d). If a rotation detection signal of the rotor 10 is not generated and rotation of the rotor is determined as failure, the correction drive pulse FP is outputted at 32 ms after the beginning of the second. Reference numeral 5 is a first detection pulse generating circuit that outputs detection pulses B6 to B12 for performing the first detection mode, based on the signal of the frequency divider circuit 2. As shown in FIG. 16(b), the detection pulses B6 to B12 are pulses of 0.125 ms width and outputted every 1 ms in a period of 6 ms to 12 ms after the beginning of the second. Reference numeral 406 is a second detection pulse generating circuit that outputs detection pulses F8 to F18 for performing the second detection mode, based on a signal of the frequency divider circuit 2. As shown in FIG. 16(c), the detection pulses F8 to F18 are pulses of 0.125 ms width and are outputted every 1 ms in a period of 8 ms to 18 ms after the beginning of the second.
Reference numeral 7 is a pulse selecting circuit that selectively outputs signals outputted from the normal drive pulse generating circuit 3, the correction drive pulse generating circuit 4, the first detection pulse generating circuit 5, and the second detection pulse generating circuit 406, based on results of the determination made in a first detection mode determination circuit 412 and a second detection mode determination circuit 413, which will be described later. Reference numeral 8 is a driver circuit that outputs a signal of the pulse selecting circuit 7 to the coil 9, rotationally drives the rotor 10, and controls rotation detection. The driver 8 outputs pulses alternately from a terminal O1 and a terminal O2 every second. Reference numeral 11 is a detection circuit that detects an induced voltage generated in the coil 9. Reference numeral 412 is the first detection mode determination circuit that performs a determination in the first detection mode, based on a detection signal of the detection circuit 11, while reference numeral 413 is the second detection mode determination circuit that performs a determination in the second detection mode, based on a detection signal of the detection circuit 11.
Note that, the detection pulses B6 to B12 are outputted to the terminal opposite to the terminal to which the normal drive pulses SP are outputted to quickly change an impedance of a closed loop including the coil 9 so that a counter-electromotive voltage generated from free vibrations of the rotor 10 after application of the normal drive pulses SP is amplified and then the detection circuit 11 detects the amplified counter-electromotive voltage. In addition, the detection pulses F8 to F18 are outputted to the same terminal as the terminal to which the normal drive pulses SP are outputted to quickly change an impedance of the closed loop including the coil 9 so that a counter-electromotive voltage generated from free vibrations of the rotor 10 after application of the normal drive pulses SP is amplified and then the detection circuit 11 detects the amplified counter-electromotive voltage.
Next, the operation of the above configuration will be described. The pulse selecting circuit 7 selects the normal drive pulses SP outputted from the normal drive pulse generating circuit 3 at the beginning of every second, and drives the step motor 20. Then, 6 ms after the beginning of the second, the first detection mode starts. In the first detection mode, the pulse selecting circuit 7 selects and outputs the detection pulses B6 to B12 outputted from the first detection pulse generating circuit 5, and controls the step motor 20 so that an impedance of the coil 9 may change. Then, the detection circuit 11 detects an induced voltage generated in the coil 9 due to the detection pulses B6 to B12. In contrast, the pulse selecting circuit 7 instructs the first detection mode determination circuit 412 to start determination operation. The first detection mode determination circuit 412 determines the presence or absence of a detection signal in the first detection mode, based on the number of inputs of the detection signals from the detection circuit 11. Based on the detection signals from the detection circuit 11, when detection signals are generated twice, the first detection mode determination circuit 412 determines detection. Immediately after this, the first detection mode determination circuit 412 instructs the pulse selecting circuit 7 to stop detection pulses outputted from the first detection pulse generating circuit 5 and to terminate the operation of the first detection mode, and makes a shift from the first detection mode to the second detection mode by instructing the second detection mode determination circuit 413 to start operation.
In the second detection mode, the pulse selecting circuit 7 selects and outputs detection pulses F8 to F18 generated by the second detection pulse generating circuit 406, and controls the step motor 20. Then, the detection circuit 11 detects an induced voltage generated in the coil 9 due to the detection pulses F8 to F18. Based on the detection signal of the detection circuit 11, when a detection signal is generated even once, the second detection mode determination circuit 413 determines that rotation is successfully performed. Immediately after this, the second detection mode determination circuit 413 controls the pulse selecting circuit 7 to terminate the detection pulses outputted from the second detection pulse generating circuit 406 so that the operation of the second detection mode is terminated, and not to output correction drive pulse FP. Here, the second detection mode determination circuit 413 terminates the detection of the detection signals generated due to the detection pulses F8 to F18 after at most six times of the detections. If no detection signal is generated during a detection trial, the second detection mode determination circuit 413 determines failure of rotation and controls the pulse selecting circuit 7 to output the correction drive pulse FP.
A method of detecting actual rotation in the above operation will be described with reference to waveform diagrams of FIGS. 16 and 17. First, a case where the rotor normally rotates will be described. FIG. 17(a) shows a current waveform induced in the coil 9 when the rotor 10 rotates. FIG. 17(b) shows a voltage waveform generated at one terminal O1 of the coil 9 in the second detection mode, and FIG. 17(c) shows a voltage waveform generated at the other terminal O2 of the coil 9 in the first detection mode. The generated waveforms of the terminals O1 and O2 are alternating pulses that reverse phases every second.
First, the normal drive pulses SP as shown in FIG. 16(a) are applied to the one end O1 of the coil 9 and the rotor 10 rotates. The waveform at this time is shown as a waveform c1 of FIG. 17(a). When the normal drive pulses SP terminate, the rotor 10 becomes in a free vibration state, and the current waveform is turned into induced current waveforms shown by c2, c3, c4. At 6 ms, the first detection mode starts, and the detection pulse B6 shown in FIG. 16(b) is applied to the coil 9. As shown in FIG. 17(a), at 6 ms, the current waveform is in the region of the current waveform c2 and a current value is on the negative side. Thus, as shown in FIG. 17(c), an induced voltage V6 generated due to the detection pulse B6 does not exceed a threshold value Vth of the detection circuit (hereinafter simply referred to as a threshold value Vth). However, at 7 ms, the current waveform enters the region of the current waveform c3, and the current value changes to the positive side. Thus, as shown in FIG. 17(c), an induced voltage V7 generated due to the detection pulse B7 is a detection signal exceeding the threshold value Vth. Similarly, also at 8 ms, the current waveform is in the region of the current waveform c3, and an induced voltage V8 generated due to the detection pulse B8 is a detection signal exceeding the threshold Vth. Since the two detection signals of the induced voltages V7, V8 have exceeded the threshold value Vth, the first detection mode switches to the second detection mode.
Since the switching to the second detection mode according to the induced voltage V8, a detection pulse of the next timing, i.e., a detection pulse F9 at 9 ms shown in FIG. 16(c) is applied to the coil 9. As shown in FIG. 17(a), at 9 ms, the current waveform is in the region of the current waveform c3 and the current value is on the positive side. Thus, as shown in FIG. 17(b), an induced voltage V9 generated due to the detection pulse F9 does not exceed the threshold value Vth. In addition, as the current waveform is still in the region of the current waveform c3, induced voltages V10, V11, V12 generated due to the respective detection pulses F10, F11, F12 do not exceed the threshold value Vth, either. However, at 13 ms, the current waveform enters the current waveform c4 shown in FIG. 17(a) and the current value changes to the negative side. Then, an induced voltage V13 generated due to the detection pulse F13 is a detection signal exceeding the threshold value Vth. Based on this detection signal, the second detection mode determination circuit 413 determines success of rotation and no correction drive pulse FP is outputted.
Next, a case where the rotor fails to rotate will be described based on the waveform diagrams of FIGS. 16 and 18. FIG. 18(a) represents a current waveform induced in the coil 9 when the rotor 10 fails to rotate because driving force of the step motor 20 drops due to a decrease of the operating voltage of the driver circuit 8, etc. FIG. 18(b) represents a voltage waveform generated at one terminal O1 of the coil 9 at this time, and FIG. 18(c) represents a voltage waveform generated at the other terminal O2 of the coil 9.
The current waveform generated at the coil when the rotor fails to rotate is the current waveform shown in FIG. 18(a). Specifically, until the end of the current waveform c1, a shown current waveform is almost the same waveform as that in the case where the rotor successfully rotates as previously mentioned. After that, however, the current waveform is turned into those shown by the current waveforms c2, c5, c6. Compared with the case where the rotor successfully rotates, the current waveform generated in the coil 9 when the rotor fails to rotate is a long and smooth waveform, as shown by the current waveform c5. Even when the rotor fails to rotate, the method of detecting rotation is the same. First, at 6 ms, the first detection mode starts and the detection pulse B6 is applied to the coil 9. As shown in FIG. 18(a), at 6 ms, the current waveform is in the region of the current waveform c2 and the current value is on the negative side. Thus, as shown in FIG. 18(c), an induced voltage V6 does not exceed the threshold value Vth. However, at 7 ms, the current waveform enters the region of the current waveform c5 and the current value changes to the positive side. Thus, as shown in FIG. 18(c), an induced voltage V7 is a detection signal exceeding the threshold value Vth. Similarly, at 8 ms, the current waveform is in the region of the current waveform c5 and an induced voltage V8 is a detection signal exceeding the threshold value Vth. Since the two detection signals of the induced voltages V7, V8 have exceeded the threshold value Vth, the first detection mode switches to the second detection mode.
Since the switching to the second detection mode according to the induced voltage V8, a detection pulse of the next timing, i.e., a detection pulse F9 at 9 ms shown in FIG. 16(c) is applied to the coil 9. As shown in FIG. 18(a), at 9 ms, the current waveform is in the region of the current waveform c5 and the current value is on the positive side. Thus, as shown in FIG. 18(b), an induced voltage V9 does not exceed the threshold value Vth. In addition, as the current waveform is still in the region of the current waveform c5, induced voltages V10 to V13 generated due to the respective detection pulses F10 to F13 do not exceed the threshold value Vth, either. Furthermore, as the current waveform is also in the region of the current waveform c5, an induced voltage V14 generated due to a detection pulse F14 for the sixth-time detections in the second detection mode does not exceed the threshold value Vth. Thus, no detection signal exceeding the threshold value is detected in a period for the six-time detections of the induced voltages V9 to V14. Hence, the second detection mode determination circuit 413 determines failure of rotation, and aborts the determination. Consequently, the pulse selecting circuit 7 selects the correction drive pulse FP, drives the step motor 20, and rotates the rotor 10 reliably. As described above, the detection of rotation or non-rotation is performed and the correction drive pulse FP can be outputted appropriately if the rotor fails to rotate.    Patent Document 1: JP-A 7-120567    Patent Document 2: JP-B 8-33457
However, in the conventional technique, detection may not be performed well in some cases where current waveforms are disturbed because a pointer having a large moment of inertia is used for a second hand or the like. The above problem will be described based on FIGS. 16 and 19. FIG. 19 is a diagram of a current waveform and a voltage waveform at a time when the rotor 10 rotates in the case where the conventional electronic clock has a hand with a large moment of inertia. FIG. 19(a) shows current waveforms induced in the coil 9 in the case where the hand with the large moment of inertia is attached. FIG. 19(b) shows voltage waveforms generated in the one terminal O1 of the coil 9 at this time, and FIG. 19(c) shows voltage waveforms generated in the other terminal O2 of the coil 9.
When a hand with a large moment of inertia is attached to the normal drive circuit, current waveforms are those shown in FIG. 19(a). Specifically, following a current waveform c1, waveforms are shaped as shown by induced current waveforms c2, c31, cx, and c41. Compared with the current waveform shown in FIG. 17(a), the stepped current waveform cx is generated between the current waveforms c31 and c41. The current waveform cx is generated because the free vibration of the rotor 10 is restricted by the moment of inertia of the second hand. For this reason, the current waveform 41 to be detected in principle shifts later. Hereinbelow, the detection operation in this case will be described. First, at 6 ms, the first detection mode starts, and the detection pulse B6 is applied to the coil 9. As shown in FIG. 19(a), at 6 ms, the current waveform is in the region of the current waveform c2 and the current value is on the negative side. Thus, as shown in FIG. 19(c), an induced voltage V6 does not exceed the threshold value Vth. However, at 7 ms, the current waveform enters the current waveform c31 and a current value changes to the positive side. Thus, as shown in FIG. 19(c), an induced voltage V7 is a detection signal exceeding the threshold value Vth. Similarly, at 8 ms, the current waveform is also in the region of the current waveform c31 and the induced voltage V8 is a detection signal exceeding the threshold value Vth. Since the two detection signals of the induced voltages V7, V8 have exceeded the threshold value Vth, the first detection mode switches to the second detection mode.
Since the switching to the second detection mode according to the induced voltage V8, a detection pulse of the next timing, i.e., a detection pulse F9 at 9 ms is applied to the coil 9. As shown in FIG. 19(a), at 9 ms, the current waveform is in the region of the current waveform c31 and the current value is on the positive side. Thus, as shown in FIG. 19(b), an induced voltage V9 does not exceed the threshold value Vth. Similarly, as the current waveform is in the region of the current waveform c31, induced voltages V10 to V12 do not exceed the threshold value Vth. At 13 ms, although the current waveform enters the current waveform cx, the current value is still on the positive side. Thus, induced voltages V13 and V14 generated due to the detection pulses F13 and F14 cannot exceed the threshold value Vth. Thus, no detection signal exceeding the threshold value is detected in a period for the six-time detections of the induced voltages V9 to V14. Hence, the second detection mode determination circuit 413 determines failure of rotation, and the pulse selecting circuit 7 selects and outputs the correction drive pulse FP. In other words, a phenomenon occurs that although the rotation has been successfully achieved, the correction drive pulse FP is outputted by the erroneous determination, thus increasing current consumption unnecessarily. The correction drive pulse FP is a pulse needing a large current consumption to drive the step motor 20 reliably. Therefore, frequent outputs of the correction drive pulse FP due to the erroneous detection leads to a problem that the battery life decreases significantly.
In order to solve the above problem, a conceivable countermeasure is to extend a determination period by simply changing the number of detection trials before the aborting of the detection in the second detection mode from six times to seven times and thus to make a determination. However, the countermeasure cannot be employed because another problem occurs as described below. The problem will be described based on FIGS. 16 and 20. FIG. 20 is a waveform diagram in the case where the driving force of the step motor 20 is weaker than that in FIG. 18, whereby the rotor 10 fails to rotate. FIG. 20(a) represents current waveforms induced in the coil 9 when the rotor 10 fails to rotate. FIG. 20(b) represents voltage waveforms generated in one terminal O1 of the coil 9 at this time, and FIG. 20(c) represents voltage waveforms generated in the other terminal O2.
FIG. 20(a) shows a current waveform generated in the coil when the rotor fails to rotate as the driving force is further weaker. Specifically, following the current waveform c1, the current waveform is tuned into those shown by the current waveforms c51, c61. In this waveform, compared with the current waveforms of FIG. 18(a), the current waveform c2 does not appear; the current waveform c51 appears following the current waveform c1; the current waveform c51 terminates early in time; and the current waveform 61 appears. In a case of such waveforms, if the detection is performed with the number of detection trials before the aborting of the detection in the second detection mode simply changed from six times to seven times, the following result is obtained. First, at 6 ms, the first detection mode starts and the detection pulse v6 is applied to the coil 9. As shown in FIG. 20(a), at 6 ms, the current waveform is in the region of the current waveform 51 and the current value is on the positive side. The induced voltage V6 is a detection signal exceeding the threshold voltage V6, as shown in FIG. 20(c). Further, at 7 ms, the current waveform is also in the region of the current waveform c51, and the induced voltage V7 is a detection signal exceeding the threshold value Vth. Since the induced voltages V6, V7 of the two detection signals have exceeded the threshold voltage, the first detection mode switches to the second detection mode.
Since the switching to the second detection mode according to the induced voltage V7, a detection pulse of the next timing, i.e., a detection pulse F8 at 8 ms is applied to the coil 9. As shown in FIG. 20(a), at 8 ms, the current waveform is in the region of the current waveform c51 and the current value is on the positive side. Thus, as shown in FIG. 20(b), an induced voltage V8 does not exceed the threshold value Vth. Further, as the current waveform is still in the current waveform of c51, the induced voltages V9 to V13 do not exceed the threshold value Vth. However, at 14 ms, which is a time for the seventh time detections in the second detection mode, the waveform enters the region of the current waveform c61 and the current value changes to the negative side. Thus, as shown in FIG. 20(b), the induced voltage V14 is a detection signal exceeding the threshold value Vth. Then, the second detection mode determination circuit 413 erroneously determines that rotation has been successfully achieved, although the rotation has been actually failed. As the pulse selecting circuit 7 does not select and output the correction drive pulse FP, the rotor does not rotate. As described above, if a period of the second detection mode is simply extended, a problem crucial for an electronic clock may occur that a step motor stops to cause a time delay.